Method and apparatus for electrostatically securing film to an object



Oct. 29, 1968 K, G, ANDER N ET AL 3,407,436

METHOD AND APPARATUS F ELECTROSTATICALLY RING FILM TO AN OBJECT SECUOriginal Filed May 6, 1964 2 Sheets-Sheet 1 Fig. 2

Fig.3

Oct. 29, 1968 G ANDERSON ET AL 3,407,436

METHOD AND APPARATUS FOR ELECTROSTATICALLY SECURING FILM TO AN OBJECTOriginal Filed May 6, 1964 2 Sheets-Sheet 2 FIG. 4

United States Patent 2 Claims. c1. 18-1) ABSTRACT OF THE DISCLOSUREApparatus for shrinking a heat shrinkable thermoplastic member about anobject comprising first and second electrodes spaced apart from eachother, means for applying a direct current potential across saidelectrodes, and means for heating a portion of the space between saidelectrodes.

This application is a division of my prior copending application Ser.No. 365,272, filed May 6, 1964, now Patent No. 3,274,302.

This invention relates to the securing of dielectric materials to anobject. In one aspect, the invention relates to restraining the movementof thermoplastic film in contact with an object. In another aspect, theinvention relates to the heat shrinking of a thermoplastic film.

It is well known to wrap or otherwise encompass objects in a wrappingfilm for the purpose of protecting the object from damage orcontamination. One of the most useful types of wrapping materials isorganic thermoplastic sheet or film. However, a problem with thesematerials is that they are difiicult to transport or convey throughautomatic wrapping equipment due to their fiimsinses. No satisfactorysolution has been found for the problem of holding this thin film inposition while being transported through the wrapping machines.Consequently, frequent wrinkling occurs resulting in a high number ofrejected products, costly shut-downs or unsightly packages. A stillfurther difficulty arises when these films are placed over objects whichare then subjected to heat to cause heat shrinking of the film aroundthe object. One specific use for this type of application is theformation of a shrink cover for containers having a lip. Again adifficulty is in the positioning of film over the container and holdingthe film during shrinking to control the shrinking, particularly when itis desired to form a tape or lock around the lip or edge or otherprojection of the object.

It is an object of the invention to provide a method and apparatus forenclosing objects in flexible dielectric materials.

It is another object of the invention to provide a method and apparatusfor restraining movement of a relatively thin, flexible, dielectricmaterial.

It is another object of the invention to provide a method and apparatusfor heat shrinking a heat shrinkable organic thermoplastic film-likemember.

It is another object of the invention to provide a method and apparatusfor preparing a cover for a container.

These and other objects of the invention will be readily apparent tothose skilled in the art from the following disclosure, claims, anddrawings.

These and other objects of the invention are broadly accomplished by theuse of electrostatic force to restrain movement of a dielectric materialplaced on an object.

It has now been found that when an object is placed between electrodeschargeable to opposite polarities and a flexible dielectric material,such as an organic thermoplastic film, is placed over the object withthe peripheral portions extending beyond the periphery of said object,that the application of a potential across the electrodes results in theelectrostatic charging of the film to a polarity opposite that of one ofthe electrodes to which the peripheral portions of the film is therebyattracted. If the object is placed on a support which is also anelectrode, then the film will be attracted to the support and willposition the film on the object.

In another embodiment of the invention, the electrostatically restrainedfilm is then subjected to sufficient heat to shrink the film with theshrink energy of the film overcoming the restraining force of theelectrostatic charge.

Suitable apparatus for carrying out the invention includes a pair ofspaced electrodes, means for applying a potential across said electrodesand means for heating the space between or around said electrodes.

It has also been found that by adjusting the electrw static force thatthe temperature required to shrink the film can be increased to atemperature above that normally required'for a heat shrinkingapplication, especially when the film is in direct and intimate contactwith the electrode.

FIGURE 1 is a schematic illustration of one embodiment of an inventionillustrating the holding of a film in place over an object.

FIGURE 2 is a schematic illustration of a second embodiment illustratingthe formation of a shrink cover around a container.

FIGURE 3 is a schematic illustration showing the film after it has beenheat shrunk to form a cover.

FIGURE 4 is a schematic illustration of one embodiment of an apparatusaccording to the invention.

The invention is useful for virtually any dielectric material, includingbut not limited to, organic thermoplastic polymeric films, sheets,netting, filaments, and the like made of polyolefins such aspolyethylene, polypropylene, and the like, vinylchloride copolymers,polyamides including nylon and the like. The term polymer as employedherein includes homopolymers, copolymers, terpolymers, block copolymers,laminates of films and the like. The invention is useful for anystructural shape so long as the material is a relatively thin flexibleelectrically non-conductive member which may be depressed by theelectrostatic force. The thickness of the material can vary depending onthe potential employed, for example, from /2 mil to 10 mils. Preferably,the dielectric material is a biaxially oriented polyoleim or polyviynl(e.g. polyethylene, polypropyene or polyvinyl chloride) film or sheet /2to 5 mils in thickness. The dielectric material may be unoriented,monodirectionally or bidirectionally oriented but, of course, will beoriented if it is to be shrunk. The material may also be irradiated suchas shown in the patents to Baird, US. 3,022,543, and Rainer et al., US.2,877,500, incorporated herein by reference.

The object to be enclosed, covered, or wrapped with the dielectricmaterial can be any shape. A preferred shape or structure, particularlyfor heat shrinking operations, is one that provides for a lip, rim, edgeor other projection on the upper surface which will permit the shrinkingfilm to removably lock around said lip, etc. A particularly usefulobject is a container having an orifice in the upper portion thereof anda lip or rim defining said orifice.

The invention is best described with reference tothe drawings.

FIGURE 1 is a schematic illustration representing the employment of theinvention for holding a film onto a surface of an object. For example,if the film is to be held onto an object passing through equipmentsubjected to air currents or the like, the object 2 may be placed ontosupport 4 which may also double as a grounded electrode. Preferably,this support comprises a substantially fiat electrically conductiveplate connected to a direct current high-voltage power source 6 which isalso connected to a second pointed electrode 8. The electrodes areconnected by suitable means to the power source so as to be of oppositepolarity. Preferably a high value resistor 10 is connected in serieswith the electrode 8 to limit current drawn to the electrodes and toprevent arcing across the gap between the electrodes. The drawingillustrates the employment of two pointed electrodes; however, the exactnumber of the electrodes is not critical so long as a uniformelectrostatic force field is generated. Selection of electrodes andtheir shape is within the skill of the art. Preferably, the chargingelectrode has a projection on it, such as a pointed electrode. The lowerelectrode 4, especially the surface thereof, may if desired be coatedwith a thin coat of a friction-reducing material such as polyethylene,polypropylene, polytetrafiuoroethylene or the like. The electrode 4 ispreferably a smooth flat surface or may be a reticular plate but in allcases it must be comprised of a conductive material such as metal, wood,paper pulp or the like. The film 12 is placed on the supported object 2and then subjected to the force field 9 thereby becoming charged to apolarity opposite that of electrode 4 which is grounded. The inventionhas been shown in perhaps its most useful form, that is with the objectresting on an electrode support, which may also be a conveyor, so thatthe film is drawn downwardly toward a support; however, it is alsowithin the scope of the invention to turn the entire operation up sidedown or in any other position so long as suitable means are provided forsupporting the object between the spaced electrodes. The film is now inposition to be subjected to any type of operation which requires thefilm to be held over the object.

The potential to be established between electrodes 4 and 8 may suitablybe in the range from about 1 to about 200, preferably from to 50,kilovolts with a current limiting resistor from 2 to 500 megohms. Thespacing between the electrodes is not particularly critical and can bevaried depending upon the voltage applied and the mode of electrodeinter-connecting. The polarity is not critical so long as theelectrostatic forces created by the opposing electrodes charge thesurface of the film 12 so as to attract the film in such a manner as toenclose the product. For example, this invention could be used forrestraining the film over an object between vertically spaced electrodesif the object is suitably supported.

FIGURE 2 is a schematic illustration of another embodiment of theinvention whereby the electrostatic forces restrain movement of the filmwhile the film is subjected to heat shrinking forces sufiicient toshrink the film about the object. The object, which in this case is bestillustrated by a container 14 having a lip 15 with a film 16 placedthereover, is placed onto an electrode support 18. The dimensions of thefilm are sufficient so that at least a portion thereof extends beyondthe periphery of the container and over the edge, preferably forsutficient distance to contact the plate 18 when depressed, unless aconductive tray is used. In addition to the use of a fiat supportingelectrode to which the film is attracted, a conductive tray or object isemployable, e.g. a metal tray, so that the film is attracted to the trayinstead of the electrode support. Again, the plate electrode 18 isconnected to a suitable high voltage direct current power source 29which is connected through a resistor 22 with a pointed electrode 24.The operation of the electrical system is essentially the same as thatdescribed for FIG- URE 1 so as to provide a force field 25, except thatthe system is simultaneously subjected to heat sufiicient to causeshrinkage of the film about the container. Since the electrostatic forcecauses attraction of the film 16 to the electrode support 18, there is apressure exerted by the charge which causes the film to resist shrinkforces and to shrink in a controlled manner. Heat may be applied by anysuitable means such as radiant heaters or by direct hot gases such ashot air as shown in FIGURE 2. This heat may be applied to any portion ofthe film by the use of a directing conduit or indirectly by means of aheat shrinking tunnel where the hot air is fed into the tunnel so as tonot disturb the film position by its kinetic energy.

This invention is particularly useful when it is required to pull theplastic film around an object to form a tape or lock thereby causing thefilm to remain in place even when additional heat energy is applied toother areas of the plastic.

Generally, the film will cover the entire upper portion of the containerbut it is sometimes desirable to only partially enclose or cover thecontainer.

In addition, this method and apparatus of heating and applyingelectrostatic pressure simultaneously can be used to heat the filmbeyond the temperature where shrink would normally occur if sufficientelectrostatic pressure is exerted.

The selection of the proper temperatures is well within the skill of theart and depends on the type of film and its orientation.

The invention is best illustrated by FIGURE 4: A 12 inch length ofinsulated tubing 35 was connected to a hot air nozzle 37 which was incommunication with a hot air blower 36 capable of supplying enough hotair to shrink irradiated biaxially oriented polyethylene film 16prepared by the method described in the patent applicatron to Baird etal. US. 3,022,543. In the center of the circular end 33 of the tube 35furtherest from the supply of the hot air 36 was mounted a sharp pointedmetal wire 32 in such a way that the hot air would blow toward andaround the point. This pointed wire 32 or electrode was connected to ahigh voltage DC. power supply 30 (polarity not important) with an outputadjustable from zero to 60 kv. at 1 ma.

A 500 megohm resistor 31 was inserted in series with the lead to theelectrode to prevent danger from electrical shock and also to preventarcing. A metal plate 18 slightly larger than the 10 inch by 10 inchfilm was connected to ground. This plate 18 also acted as a support forthe object to be wrapped and for the sheet of irradiated biaxiallyoriented polyethylene film 16 which was draped over the object.

A number of trays 14 of a variety of shapes, some being electricalconductors and others being made from insulating material, wereobtained.

Blocks of wood were inserted into the trays 14 to simulate a product.The hot air nozzle containing the electrode 32 was held from 2 inches to10 inches above the tray 14 which had been draped with film 16 and movedin a circular motion around the tray. A voltage of approximately 40 kv.was applied to the electrode.

The electrostatic pressure caused the film 16 to adhere to the metalplate 18, however, the shrink energy of the film was sufficient, in thiscase, to cause the film to slip on the plate and drag inwardly aroundthe base of the tray 14. Since the trays 14 were fitted with a lip 15around the circumference, the film was caused to mechanically lockaround this lip.

While certain examples, structures, composition and process steps havebeen described for purposes of illustration, the invention is notlimited to these. Variation and modification within the scope of thedisclosure and the cliams can readily be effected by those skilled inthe art.

We claim:

1. Apparatus for shrinking a heat shrinkable thermoplastic member aboutan object comprising, in combination:

(a) a first upper pointed electrode,

(b) a second substantially flat lower electrode support for saidcontainer spaced apart from (a),

(0) means for applying a direct current electrical potential across saidelectrodes and,

(d) means independent of said electrodes for simultaneously directing aheated gas from above into the space between said electrodes whileapplying said potential.

2. The apparatus of claim 1 wherein said means for heating comprises aconduit connected to a source of hot gas axially positioned relative tosaid first electrode and directed toward said second electrode.

UNITED 6 References Cited STATES PATENTS Gladding 188 Norton 18-8 Tillet al. 188 XR Peel et a1. 18- 6 XR Bennett et al. 18--6 Klingaman219-6.5 XR Simorda 18-6 Anderson 2196.5 XR Wupperman 219-10 WILLIAM J.STEPHENSON, Primary Examiner.

